For one thing, the guest's service fee would be a lot higher than $21. That's *your* service fee. (3% of $690 is $20.70, which AirBnB rounds up to $21.) The guest's service fee is more likely 10-12%, so $69-83. I know that still doesn't explain the discrepancy, but without knowing the dates the guest is trying to book, that's as much as I can figure out.
Is the $711 that you see your "payout"? If so, keep in mind that your payout (after your 3% fee) will never match the guest's full charge (which includes their 6-12% service fee). All that said, the discrepancy you mention still seems like something is wrong. If you do figure this out, I'd be interested in hearing the details.